1. Field of the Invention
The invention relates to alignment measurement in lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) of a substrate (e.g. a silicon wafer). Transfer of the pattern includes imaging the pattern onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include “steppers”, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and “scanners”, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It is important that a substrate is properly aligned with the projection system and the patterning device. In practice, multiple patterns may be projected on top of each other to obtain a three dimensional product structure. These multiple patterns should be properly aligned with respect to each other to obtain a reliable product structure. This overlay requirement is increasingly important with increasing resolution.
Several methods and measurement tools are available to measure alignment of the substrate with respect to the projection system and/or the patterning device. Known alignment sensors may use multiple different wavelengths, each with a specific polarization direction. The benefit of using polarization directions is evident when the alignment marks are sub-segmented to create polar marks. These polar marks may use different sub-segmentations in the lines and the spaces of the alignment mark to increase the optical contrast between the lines and the spaces.
Improving alignment by combining multiple colors is starting to be used, for example by using ‘smooth-color dynamic’ alignment recipes or by doing ‘color to color’ analysis. These methods aim to improve the measured align position by minimizing the effect of mark asymmetry, but can't improve the signal to noise ratio of a single measurement.
In certain cases, such as shallow marks, or marks with large background signals, it might even be required to improve the signal to noise ratio in order to get accurate alignment results. In such cases a measurement beam with a single wavelength may be preferred, since the reflectivity of the rest of the stack is wavelength dependant.
Different colors also will be produced by different light sources. Although combining different colors reduces the influence of laser noise on the alignment signal, it cannot completely eliminate the influence.